refactor(mobile): guard empty pool in pickRandom and parameterise its test

Address review feedback:
- Throw on an empty pool so the `T` return type is honoured at runtime.
- Convert the index-to-element test to an `it.each` table per repo preference.

Generated-By: PostHog Code
Task-Id: cddb89a6-66c3-4e63-b386-f8eb0394865f

Author: Gilbert09

apps/mobile/src/lib/pickRandom.test.ts

@@ -13,10 +13,15 @@ describe("pickRandom", () => {
     }
   });
 
-  it("draws a fresh pick from the random value on each call", () => {
-    const pool = ["a", "b", "c", "d"];
-    vi.spyOn(Math, "random").mockReturnValueOnce(0).mockReturnValueOnce(0.99);
-    expect(pickRandom(pool)).toBe("a");
-    expect(pickRandom(pool)).toBe("d");
-  });
+  it.each([
+    { randomValue: 0, expected: "a" },
+    { randomValue: 0.99, expected: "d" },
+  ])(
+    "returns $expected when Math.random returns $randomValue",
+    ({ randomValue, expected }) => {
+      const pool = ["a", "b", "c", "d"];
+      vi.spyOn(Math, "random").mockReturnValueOnce(randomValue);
+      expect(pickRandom(pool)).toBe(expected);
+    },
+  );
 });

apps/mobile/src/lib/pickRandom.ts

@@ -1,3 +1,4 @@
 export function pickRandom<T>(pool: readonly T[]): T {
+  if (pool.length === 0) throw new Error("pickRandom: pool must not be empty");
   return pool[Math.floor(Math.random() * pool.length)];
 }